Enantiopure Dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophenes: Reaching High Magnetoresistance Effect in OFETs.

Chiral molecules are known to behave as spin filters due to the chiral induced spin selectivity (CISS) effect. Chirality can be implemented in molecular semiconductors in order to study the role of the CISS effect in charge transport and to find new materials for spintronic applications. In this study, the design and synthesis of a new class of enantiopure chiral organic semiconductors based on the well-known dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) core functionalized with chiral alkyl side chains is presented. When introduced in an organic field-effect transistor (OFET) with magnetic contacts, the two enantiomers, (R)-DNTT and (S)-DNTT, show an opposite behavior with respect to the relative direction of the magnetization of the contacts, oriented by an external magnetic field. Each enantiomer displays an unexpectedly high magnetoresistance over one preferred orientation of the spin current injected from the magnetic contacts. The result is the first reported OFET in which the current can be switched on and off upon inversion of the direction of the applied external magnetic field. This work contributes to the general understanding of the CISS effect and opens new avenues for the introduction of organic materials in spintronic devices.

Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors.

The charge transport of crystalline organic semiconductors is limited by dynamic disorder that tends to localize charges. It is the main hurdle to overcome in order to significantly increase charge carrier mobility. An innovative design that combines a chemical structure based on sulfur-rich thienoacene with a solid-state herringbone (HB) packing is proposed and the synthesis, physicochemical characterization, and charge transport properties of two new thienoacenes bearing a central tetrathienyl core fused with two external naphthyl rings: naphtho[2,3-b]thieno-[2''',3''':4'',5'']thieno[2″,3″:4',5']thieno[3',2'-b]naphtho[2,3-b]thiophene (DN4T) and naphtho[1,2-b]thieno-[2''',3''':4'',5'']thieno[2'',3'':4',5']thieno[3',2'-b]naphtho[1,2-b]thiophene are presented. Both compounds crystallize with a HB pattern structure and present transfer integrals ranging from 33 to 99 meV (for the former) within the HB plane of charge transport. Molecular dynamics simulations point toward an efficient resilience of the transfer integrals to the intermolecular sliding motion commonly responsible for strong variations of the electronic coupling in the crystal. Best device performances are reached with DN4T with hole mobility up to µ = 2.1 cm2 V-1 s-1 in polycrystalline organic field effect transistors, showing the effectiveness of the electronic coupling enabled by the new aromatic core. These promising results pave the way to the design of high-performing materials based on this new thienoacene, notably through the introduction of alkyl side-chains.

Unraveling the nanoscale morphologies of mesoporous perovskite solar cells and their correlation to device performance.

Hybrid solar cells based on organometal halide perovskite absorbers have recently emerged as promising class for cost- and energy-efficient photovoltaics. So far, unraveling the morphology of the different materials within the nanostructured absorber layer has not been accomplished. Here, we present the first visualization of the mesoporous absorber layer in a perovskite solar cell from analytical transmission electron microscopy studies. Material contrast is achieved by electron spectroscopic imaging. We found that infiltration of the hole transport material into the scaffold is low and inhomogeneous. Furthermore, our data suggest that the device performance is strongly affected by the morphology of the TiO2 scaffold with a fine grained structure being disadvantageous.

Determination of Singlet Exciton Diffusion Length in Thin Evaporated C60 Films for Photovoltaics.

C60 is used as an electron acceptor in small molecule photovoltaic devices in combination with various electron donors. The transport of excitons, i.e., bound electron/hole pairs, is an important factor determining the efficiency of such devices. Here we investigate the exciton diffusion length in C60 with the electrodeless time-resolved microwave conductance (TRMC) technique. Bilayers of 30 nm Zn-phthalocyanine with a C60 layer with variable thickness are prepared by physical vapor deposition. Analysis of the photoconductance with an exciton diffusion model yields a diffusion length of 7 nm, and the mobility of holes along Zn-Phthalocyanine stacks is close to 1 cm(2)/(V s). From analysis of the rise and decay of the TRMC transients, we attribute the photoconductance to diffusion and dissociation of singlet excitons rather than triplets. The energy transfer rate between C60 molecules exceeds 8 × 10(10) s(-1). Exciton diffusion cannot be described by the Förster model due to the close proximity of the molecules.

Parallel bulk-heterojunction solar cell by electrostatically driven phase separation.

Thermal co-evaporation of C(60) fullerene and two merocyanine dyes affords bulk heterojunction solar cells with improved short-circuit currents and power conversion efficiencies in comparison with the respective single donor cells. These results are rationalized by the formation of three distinct subphases driven by differences in molecular shape and electrostatic interactions.

High-performance air-stable n-channel organic thin film transistors based on halogenated perylene bisimide semiconductors.

The syntheses and comprehensive characterization of 14 organic semiconductors based on perylene bisimide (PBI) dyes that are equipped with up to four halogen substituents in the bay area of the perylene core and five different highly fluorinated imide substituents are described. The influence of the substituents on the LUMO level and the solid state packing of PBIs was examined by cyclic voltammetry and single crystal structure analyses of seven PBI derivatives, respectively. Top-contact/bottom-gate organic thin film transistor (OTFT) devices were constructed by vacuum deposition of these PBIs on SiO(2) gate dielectrics that had been pretreated with n-octadecyl triethoxysilane in vapor phase (OTS-V) or solution phase (OTS-S). The electrical characterization of all devices was accomplished in a nitrogen atmosphere as well as in air, and the structural features of thin films were explored by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM). Several of those PBIs that bear only hydrogen or up to two fluorine substitutents at the concomitantly flat PBI core afforded excellent n-channel transistors, in particular, on OTS-S substrate and even in air (mu > 0.5 cm(2) V(-1) s(-1); I(on)/I(off) > 10(6)). The best OTFTs were obtained for 2,2,3,3,4,4,4-heptafluorobutyl-substituted PBI 1a ("PTCDI-C4F7") on OTS-S with n-channel field effect mobilities consistently >1 cm(2) V(-1) s(-1) and on-to-off current rations of 10(6) in a nitrogen atmosphere and in air. For distorted core-tetrahalogenated (fluorine, chlorine, or bromine) PBIs, less advantageous solid state packing properties were found and high performance OTFTs were obtained from only one tetrachlorinated derivative (2d on OTS-S). The excellent on-to-off current modulation combined with high mobility in air makes these PBIs suitable for a wide range of practical applications.

Structure determination of seven phases and solvates of Pigment Yellow 183 and Pigment Yellow 191 from X-ray powder and single-crystal data.

The crystal structures of two industrially produced laked yellow pigments, Pigment Yellow 183 [P.Y. 183, Ca(C16H10Cl2N4O7S2), alpha phase] and Pigment Yellow 191 [P.Y. 191, Ca(C17H13ClN4O7S2), alpha and beta phases], were determined from laboratory X-ray powder diffraction data. The coordinates of the molecular fragments of the crystal structures were found by means of real-space methods (simulated annealing) with the program DASH. The coordinates of the calcium ions and the water molecules were determined by combining real-space methods (DASH and MRIA) and repeated Rietveld refinements (TOPAS) of the partially finished crystal structures. TOPAS was also used for the final Rietveld refinements. The crystal structure of beta-P.Y. 183 was determined from single-crystal data. The alpha phases of the two pigments are isostructural, whereas the beta phases are not. All four phases exhibit a double-layer structure, built from nonpolar layers containing the C/N backbone and polar layers containing the calcium ions, sulfonate groups and water molecules. Furthermore, the crystal structures of an N,N-dimethylformamide solvate of P.Y. 183, and of P.Y. 191 solvates with N,N-dimethylformamide and N,N-dimethylacetamide were determined by single-crystal X-ray analysis.

Rainbow perylene monoimides: easy control of optical properties.

Perylene dyes have been widely used as photoreceptors in organic photovoltaics because of their outstanding photo-, thermal and chemical stability as well as their excellent photophysical properties. Herein we describe a novel generation of perylene dyes based on N-(2,6-diisopropylphenyl)-perylene-3,4-dicarboximide. The optical properties of these novel perylenes can be finely tuned via the substituents in the 1-, 6- and 9-positions of the perylene core. The facile synthesis, tunable orbital and absorption properties, and electrochemical potentials help us to design efficient perylene sensitizers for solar-cell applications.

Novel core-expanded rylenebis(dicarboximide) dyes bearing pentacene units: facile synthesis and photophysical properties.

Two synthetic routes for the benzannulation in the "bay"-region of rylenebis(dicarboximide)s leading to new pi-system-expanded chromophores are described. The first route follows a two-step approach: Suzuki coupling of bromo-substituted perylenebis(dicarboximide) with 2-bromophenylboronic acid, followed by palladium-catalysed dehydrobromination. The second route is best described as a palladium-assisted cycloaddition of benzyne, formed in situ, to the bay-region of the bromo-substituted rylene core. Two new types of core-expanded rylene dyes were synthesised: yellow dibenzocoronenebis(dicarboximide)s, absorbing at 490 nm, and a green dinaphthoquaterrylenebis(dicarboximide), which absorbs at 700 nm. These new chromophores are characterised by significant hypsochromic shifts of absorption, compared to their parent rylenebis(dicarboximide)s, excellent photostabilities and high fluorescence quantum yields.

Solution-assisted assembly of organic semiconducting single crystals on surfaces with patterned wettability.

Two efficient approaches to assembling organic semiconducting single crystals are described. The methods rely on solvent wetting and dewetting on substrates with patterned wettability to selectively direct the deposition or removal of organic crystals. Substrates were functionalized with different self-assembled monolayers (SAMs) to achieve the desired wettabilities. The assembly of different organic crystals over centimeter-squared areas on Au, SiO2, and flexible plastic substrates was demonstrated. By designing line features on the substrate, the alignment of crystals, such as CuPc needles, was also achieved. As a demonstration of the potential application of this assembly approach, arrays of single-crystal organic field-effect transistors were fabricated by patterning organic single crystals directly onto and between transistor source and drain electrodes.

Diiminoisoindoline: tautomerism, conformations, and polymorphism.

Two polymorphs of the industrially important compound diiminoisoindoline occur in the amino tautomeric form as a conformational isomorph with a 1 : 1 mixture of the syn- and anti-isomers, and a conformational polymorph containing only the syn-isomer.

Crystal structure prediction of small organic molecules: a second blind test.

The first collaborative workshop on crystal structure prediction (CSP1999) has been followed by a second workshop (CSP2001) held at the Cambridge Crystallographic Data Centre. The 17 participants were given only the chemical diagram for three organic molecules and were invited to test their prediction programs within a range of named common space groups. Several different computer programs were used, using the methodology wherein a molecular model is used to construct theoretical crystal structures in given space groups, and prediction is usually based on the minimum calculated lattice energy. A maximum of three predictions were allowed per molecule. The results showed two correct predictions for the first molecule, four for the second molecule and none for the third molecule (which had torsional flexibility). The correct structure was often present in the sorted low-energy lists from the participants but at a ranking position greater than three. The use of non-indexed powder diffraction data was investigated in a secondary test, after completion of the ab initio submissions. Although no one method can be said to be completely reliable, this workshop gives an objective measure of the success and failure of current methodologies.